my f5 is showing sync recommended message (on both active and standby), but i havent done any changes to it.how to troubleshoot?

Hopefully you have auditing turned on and can check the audit logs. They are located at /var/log/audit or audit.1.gz, audit.2.gz, etc. Otherwise you can look at the dashboard (if you are running I think 11.4 or later) and change the time to month. The little red balls at the bottom with designate days with changes and what was changed.